1. Field of the Invention
The present invention relates to the field of digital image processing and, more specifically, to methods of identification or authentication based on digital images of an eye.
2. Discussion of the Related Art
Iris recognition is a well tested biometric identification technique, provided that the image on which the analysis and identification methods are applied is an exploitable image. In particular, the performance of recognition algorithms strongly depends on the definition of the image of the iris to be identified.
Currently, in most applications, and especially in on-board applications (for example for an access control of a telephone or a laptop computer, for an electronic key, etc.), the used camera (digital sensor and lens) does not have an autofocus system adjusting the (real or simulated) focal distance according to the distance.
Further, for obtaining a good resolution of the iris which only takes up a small surface area of the eye, the images are taken at a relatively short distance (generally on the order of from 10 to 30 cm). This results in a small field depth (distance range between the camera and the eye in which the image is clear). This small field depth added to the fact that the eye is spherical may even generate definition differences between areas of a same eye image.
A processing previous to the actual iris recognition thus consists of selecting a sufficiently clear image.
Generally, the shooting device takes a number of images ranging between 5 and 50 and a pre-processing system selects the image to be submitted to the actual recognition algorithm.
The definition evaluation amounts to assigning, to each image, a score characteristic of its definition. This enables either selecting a sufficiently clear image with respect to a determined threshold, or selecting the clearest image among the images of a set. By convention, the higher the score assigned to an image, the clearer the image.
Various techniques for evaluating the definition of digital images have already been provided, be it based on a filtering, a wavelet transformation (WO-A-00/36551), or a frequency analysis (WO-A-00/30525).
All these techniques have the common disadvantage of being slow, especially if they are implemented in miniaturized products where the processing capacity is limited (electronic key, for example). “Slow” means that they are poorly compatible with a real time processing of images taken at a rate greater than 10 images per second. The need for rapidity is, in on-board applications, linked to the need for identification or authentication rapidity of a user by its iris, where the selection of a clear image thereof is a previous step.
Another disadvantage is the complexity in terms of size of the program necessary to execute the definition evaluation algorithm.
Another problem is, to save time and complexity of the method, to limit the area to be examined in definition. In particular, the small field depth in addition to the fact that the eye is spherical and that elements such as eyelashes may be included in the image makes this area localization important to evaluate the definition of the iris and not that of other image areas.
Another problem which is posed for the definition determination of iris images, or more generally of a specific area of an image taken with a small field depth and acquired at small distance, is linked to the presence of areas external to the area to be evaluated (for example, eyelashes), which may be clear while the iris is not. This problem is especially present in operators or algorithms taking into account luminosity gradients, which amounts to taking more account of the contours than of the actual areas. In particular, this is a disadvantage of a conventional operator or algorithm known as an FSWM operator which is besides known as an operator providing acceptable results.
Another problem which is also posed for the definition evaluation of image areas taken at small distance and with a small field depth is linked to the necessary illumination of the taken subject. For eye image sensors, it generally is a light-emitting diode. This light source creates specular spots which pollute the definition evaluation. In particular, the FSWN operator mentioned hereabove may be deceived by the presence of specular spots which tend to mask luminosity gradients originating from the iris with more significant gradients originating from the spots.